1. Field of the Invention
The present invention relates to wavelength division multiplexers (WDMs) and, in particular, to WDMs utilizing interferometric devices capable of channel separation and routing.
2. Discussion of the Related Art
With existing fiber optic networks, there is often the need to increase information transmission capacity. However, both physical and economic constraints can limit the feasibility of increasing transmission capacity. For example, installing additional fiber optic cable to support additional signal channels can be cost prohibitive, and electronic system components may impose physical limitations on the speed of information that can be transmitted. The use of wavelength division multiplexers (WDMs) provides a simple and economical way to increase the transmission capacity of fiber optic communication systems by allowing multiple carrier wavelengths (channels) to be transmitted and received over a single optical fiber through signal wavelength multiplexing and demultiplexing. In addition, WDMs can be used in fiber optic communication systems for other purposes, such as dispersion compensation, noise reduction, and gain flatting, i.e., maintaining a uniform gain within the usable bandwidth for erbium-doped amplifiers.
WDMs can be manufactured using, for example, biconical tapered fusion (BTF) technology. Typically, two optical fibers are fused together along an interior portion to form a fused-fiber coupler, so that light of two wavelengths (e.g., 1310 nm and 1550 nm) entering the input terminals of the first and second fibers, respectively, are multiplexed onto a single fiber. The coupling ratios for the two channels (the signals at 1310 nm and 1550 nm) exhibit complementary sinusoidal behavior for amplitude as a function of frequency within the passband of the WDM, with each channel having one or more peaks (or windows) within the passband. Information carried by the two signals along the single fiber is then demultiplexed at the WDM outputs.
Multi-window WDMs (MWDMs) have two or more peaks of amplitude as a function of frequency (or operational windows) for each channel within a passband. MWDMs can also be made using BTF technology by twisting two optical fibers together, fusing the center portion together, and pulling the fibers until a desired multi-window transmission spectrum appears at a monitored fiber output terminal. Such a long-tapered-fusing technology is discussed in commonly-owned U.S. Pat. No. 5,809,190 referenced above.
Even though optical fibers have high information carrying capacity, the overall optical communication link may be restricted by practical bandwidth considerations, thereby limiting the size of passbands. Therefore, to increase the efficiency of bandwidth use, the passband should contain as many communication channels or windows as possible, subject to constraints with the system. WDMs have interferometric devices at the input of the communication system to combine (multiplex) the wavelengths of multiple optical signals into a single carrier signal for transmission, and interferometric devices at the system output to separate (demultiplex) the wavelengths back to individual signal streams. Suitable interferometric devices are ones having the wavelength combination/separation and routing capabilities, such as Mach-Zehnder Interferometers (MZIs), couplers, fiber gratings, and array waveguides.
The wavelengths of the various channels in the useable bandwidth are filtered to pass the desired wavelengths and reject signals outside the wavelength peaks. To increase the number of channels or windows, the bandwidth of each channel or window and/or the separation between wavelength peaks should be minimized. However, associated with each wavelength peak are sidelobes on either side of the peak. These sidelobes can be significant in power, and if the channel separation is small, the sidelobes of adjacent and neighboring channels can cause significant cross-talk between pass and stop channels. As a result, communication quality and channel selectivity for the system is degraded.
Accordingly, it is desired to have a wavelength division multiplexer that is capable of suppressing sidelobes in a multi-channel spectrum.